1. Field of the Invention
The present invention relates generally to radio positioning and communication systems, and is particularly concerned with a radio position determination and message transfer system in which one or more artificial satellites are employed as relay stations for the ranging and message signals.
2. Description of the Prior Art
The present invention constitutes an improvement or modification of the satellite-based position determining system described in U.S. Pat. No. 4,359,733, issued to G. K. O'Neill on Nov. 16, 1982. In that system, the user vehicles are equipped with transponders for transmitting a uniquely coded beacon or reply signal in response to a received general interrogation signal. Three repeater-carrying satellites are provided at spaced orbital locations above the earth for receiving and retransmitting the reply signals produced by the vehicle transponders. A ground station periodically transmits the general interrogation signal, and also receives and processes the reply signals retransmitted by the three satellites in order to determine the instantaneous positions of the user vehicles.
In order to avoid signal overlap and equipment saturation at the ground station, each vehicle transponder in U.S. Pat. No. 4,359,733 includes means responsive to the general interrogation signal for inhibiting the response of the transponders to subsequent general interrogation signals for a predetermined time interval following the response of the transponder to a first general interrogation signal. This avoids the need for discrete addressing of individual transponders, time-slotted polling, multiple frequencies, and the various other complex techniques which have previously been considered necessary to reduce signal overlap at the receiving station. In addition, the possibility of varying the inhibit interval allows the effective response rate to be modified for different classes of users, or for the same user during different periods of need, without changing the actual interrogation rate at the ground station.
The system described in U.S. Pat. No. 4,359,733 is effective not only to receive ranging signals from the user vehicles for position computation purposes, but also to relay messages between the ground station and the user vehicles. These messages may consist, for example, of emergency alerts sent from the user vehicles to the ground station, or of computed position or navigational information sent from the ground station to the user vehicles. In the case of inbound messages sent from the user vehicles to the ground station, the message data is keyed into the user's equipment and sent out as part of the user's response to the next interrogation signal. In the case of outbound messages sent from the ground station to the user, the message data includes a destination address specifying the particular user for which the message is intended.
For certain classes of users, such as aircraft, there is not usually a problem with regard to the need for line-of-sight communication with the satellites. For other classes of users, however, the quality of the communication link to the satellites may not be assured at all times. Particularly in the case of land vehicles in motion, there is a basic problem in that the vehicle may be passing through or near obstructions, such as buildings, dense foliage, tunnels, and so on, where the communication link quality becomes too poor for transmission. If the user attempts to transmit a message under these conditions, or if the ground station attempts to send a message to the user, the message will not get through and must be repeated. At the user level, the message retry consumes additional power and is therefore disadvantageous in the case of battery-operated user equipment. A message retry by the ground station is also disadvantageous since it wastes power in the satellite and increases the total signal traffic.